This invention relates to radiofrequency ablation of human tissue, particularly heart tissue for the elimination of arrhythmias.
The following references provide useful background material to the present invention, and are all incorporated herein by reference.
Patent References:Kushihashi12/1969 3,484,263Levy6,19703,515,571Avitall1/19965,487,385Swanson12/1996 5,582,609Swartz12/1998 5,846,223Shearon7/19995,919,188Haissauguerre5/20006,064,902Amundson1/20016,178,346O'Brian1/20016,168,825Suorsa3/20016,206,831Webster4/20016,210,406Tu5/20016,238,390Gaiser6/20016,241,728Stewart12/2001 6,325,797Sutton9/20026,443,950Amundson11/2002 PCT/US02/361;PCT/US02/364Other ReferencesKnight5/2004Heart Rhythm 2004Abstract 26Lardo et al. “Visualization and Temporal/Spatial Characterization of Cardiac Radiofrequency Ablation Lesions using Magnetic Resonance Imaging” Circ 2000: 102: 698-705
In the field of cardiology, arrhythmias (irregularities in heart rate) are increasingly being treated by a procedure called catheter ablation. In catheter ablation, a catheter is inserted, usually from the femoral veins, into the right heart of a patient, where it is critically positioned to ablate spots in the heart, thought to be propagating the arrhythmia. If the ablation is successful, the arrhythmia is permanently disrupted and the patient no longer requires conventional therapy such as drugs or implanted devices such as pacemakers or defibrillators. For example, aberrant conduction pathways between atria and ventricles create some pathological high heart rates, called supraventricular tachyarrhythmias (SVT's). These pathways are detected by mapping electrical potentials with multi-electrode catheters in the atrium. Once located, a small radio-frequency burn of about 10 square millimeters is created, electrically ablating the pathway.
A conventional RF ablation catheter contains a platinum hemisphere on the distal end of the catheter. The electrode is about 2.3 mm in diameter and from 4-10 mm in length. Ablation occurs in tissue because the radiofrequency energy heats the intracellular fluid inside the cell, causing the cell to desiccate. It is very difficult to determine how much of the catheter electrode is in contact with the tissue since catheter electrodes and probes are cylindrical and the heart tissue cannot be visualized clearly with existing fluoroscopic technology. Further, because of the cylindrical shape, some of the exposed electrode or probe area will almost always be in contact with blood circulating in the heart, giving rise to a risk of clot formation.
Clot formation is frequently associated with RF energy inside the heart because it is difficult to prevent the blood from being exposed to the electrode or probe surface. Some of the RF current flows through the blood between the electrode and the heart tissue and this blood is coagulated possibly resulting in clot formation. When RF energy is applied, the temperature of the electrode is typically monitored so as to not exceed a preset level, but temperatures necessary to achieve tissue ablation frequently results in blood coagulum forming on the electrode. Overheating of tissue is a major complication, because the temperature monitoring only gives the temperature of the electrode or probe, which is being cooled on the outside by blood flow. The actual temperature of the tissue being ablated by the electrode or probe can be considerably higher than the electrode or probe temperature, and this can result in overheating, or even charring, of the tissue. Overheated or charred tissue can act as a locus for thrombus and clot formation. When applied to the left heart as in atrial fibrillation eradication, the thrombus can lead to strokes and heart attacks.
The electrode temperature is only an indirect measure of the complex temporal and spatial temperature at the electrode-tissue interface. It can significantly underestimate the temperature at the tissue interface because of cooling effect of flowing blood. It provides only an average temperature of the blood and tissue contacting the electrode with no information concerning the spatial temperature profile. The relatively large thermal mass of the electrode delays the temperature changes occurring at the electrode-tissue interface. Ideally, a multitude of discrete thermocouples or other temperature measuring devices on the electrode surface would provide a much better measure of the tissue and blood temperature properties.
More common arrhythmias such as atrial fibrillation, flutter and more potentially lethal arrhythmias such as post-myocardial-infarct ventricular tachycardia require lines to be burned instead of “spots”. Atrial fibrillation is the most common arrhythmia in man, affecting over 3 million people in the United States. In this arrhythmia, the atria quiver; no longer pumping blood, and an unstable heart rate is a side feature. Patients with AF are much more prone to stroke, congestive heart failure, myocardial infarctions and fatal ventricular arrhythmias. Patients can be in temporary (paroxysmal) atrial fibrillation or permanent atrial fibrillation (most dangerous). Atrial flutter, often a precursor to atrial fibrillation, is a fluttering of the atria, also with loss of atrial mechanical function. It has a prevalence ranging from 1 in 81 to 1 in 238 hospitalized patients. This arrhythmia is usually disabling and resistant to antiarrhythmic drugs and it carries a potential risk of thromboembolism and cardiomyopathy. Post-myocardial-infarct ventricular tachycardia occurs following a myocardial infarction. The infarct sometimes results in short-circuiting of the ventricular electrical activation pattern, resulting in tachycardia. It is a frequently lethal tachycardia and as a result is the principle indication for receiving an implantable defibrillator.
Unlike SVT's, ablation lines, rather than spots, need to be created to eradicate these arrhythmias, based on anatomical considerations rather than electrical potentials. For atrial fibrillation, lines circling the pulmonary veins and sometimes-additional lines seem to be effective for the eradication of the arrhythmia. For atrial flutter, a linear ablation around the tricuspid annulus and Eustachian valve and ridge on the septum is effective in terminating the arrhythmia. For post-myocardial-infarct ventricular tachycardia, a circular ablation around the infarct is sometimes successful in eradicating the arrhythmia.
Since these procedures are performed without local visualization, the location of the burns cannot be seen; making connection of the spots very difficult. As stated in Lardo et al. Visualization and Temporal/Spatial Characterization of Cardiac Radiofrequency Ablation Lesions using Magnetic Resonance Imaging Circ 2000: 102:698-705, “ . . . There is general agreement that new approaches to facilitate anatomy-based catheter ablation are needed”
Catheter ablation of atrial fibrillation is currently accomplished by accessing the left atrium through a needle puncture from the right atrium, and placing circular lesions around the pulmonary veins. Various circular burn configurations have been evaluated, ranging from encircling all of the pulmonary veins to encircling each one individually. Some protocols also advocate the placing of additional linear lesions between the pulmonary vein and the mitral valve. A dangerous complication of this procedure is stenosis of the pulmonary veins from ablations too far inside the pulmonary veins.
Various ablation catheters have been developed which attempt to produce continuous lesions. Avitall (U.S. Pat. No. 5,487,385), Kroll (U.S. Pat. No. 6,287,306), Tu (U.S. Pat. No. 6,241,728 ) and Shearon (U.S. Pat. No. 5,919,188) disclose catheters thought to produce linear lesions. Sutton (U.S. Pat. No. 6,443,950) and Swartz (U.S. Pat. No. 5,846,223) disclose catheters, which are intended to produce continuous lesions for atrial flutter eradication. Catheters capable of forming linear circular lesions, needed for pulmonary vein isolation, are disclosed by Haissauguerre (U.S. Pat. No. 6,064,902), Tu (U.S. Pat. No. 6,241,728), Stewart (U.S. Pat. No. 6,325,797) and Gaiser (U.S. Pat. No. 6,241,728). All of these catheters rely on spatial configurations to orient the catheter in close proximity to the targeted the tissue and electrode separations small enough so that the individual lesions form one continuous lesion. For example, Stewart (U.S. Pat. No. 6,325,797) teaches a catheter of closely spaced electrodes where the distal end assumes a circular configuration for placement around a pulmonary vein. Haissauguerre (U.S. Pat. No. 6,064,902) teaches a pulmonary vein ablation catheter which is inserted into the pulmonary vein and rotated during ablation to produce a linear lesion.
In general, these linear-lesion producing catheters have two problems: variations in cardiac anatomy and inability to assess lesion production. If the cardiac area to be ablated conforms to the shape of the lead and all of the ablation electrodes are in intimal contact with the tissue, a linear lesion at the proper location should be formed. However, there is great variation in cardiac anatomy among patients. For example, most patients have four pulmonary veins, however some patient's have more veins. Some patients have pulmonary veins in close proximity to each other rather than being spatially separate. If a circular configured catheter, such as Stewart (U.S. Pat. No. 6,325,797), were used in pulmonary veins in close proximity to each other, some of the electrodes might actually reside in the neighboring pulmonary vein, possibly causing pulmonary vein stenosis.
Producing a continuous lesion by connecting individual spot lesions is also somewhat speculative, since the contact pressure against tissue determines the size of the lesion. Catheter configurations such as Swanson (U.S. Pat. No. 5,582,609), which form a linear lesion from the connection of small circular lesions, use electrode separations that produce a linear lesion if the electrodes are lying against tissue. If an electrode is not lying against tissue, a much smaller lesion or no lesion will be formed, leaving a corresponding gap in the linear lesion. Gaps in linear lesions may actually worsen the arrhythmogenic condition, such as in atrial flutter ablation, where gaps in the lesion can lead to atrial fibrillation.
The difficulty of making continuous lesions with radio-frequency energy has led to the exploration of other ablation sources. Sources such as lasers, microwaves, ultrasonic energy and freezing have all been proposed by investigators as a means of making linear lesions. The safety and efficacy of these approaches is still unclear. For example, laser ablation is a common technique in other areas of medicine, where it is possible to image the effects of the ablation. When performed blindly, however, laser ablation can lead to perforation of a cardiac chamber.
Intracardiac echocardiography uses ultrasound to indirectly visualize the heart and electrode tissue interface during catheter ablation procedures. This imaging technique is limited by the need for an additional catheter to be placed in the heart. In addition it is limited by the need for frequent repositioning of the intracardiac echo catheter in order to keep the tip of the ablation catheter within the field of view. Cardiac motion makes continuous imaging of the interface between the ablation electrode and the endocardium difficult. As a separate catheter, the ICE catheter can practically only be positioned in the near vicinity of the ablation catheter. Suorsa (U.S. Pat. No. 6,206,831) discloses a more sensitive ultrasound means of evaluating tissue contact by having the ultrasound transducers adjacent to each of the electrodes. The patent assumes that if the electrodes have a certain separation and the ultrasonic transducers verify tissue contact, then a continuous lesion will result. There is no discussion of imaging through the electrode tip to provide close proximity images of the tissue surface.
Recently, a technique called infrared endoscopy has been developed, using near-infrared light of creating a direct image of structures through blood by Amundson (U.S. Pat. No. 6,178,346). This device illuminates tissue with infrared light. The patent discusses and makes claims for illuminating structures obscured by blood with infrared illumination and recording the reflected image in an infrared camera. Illumination wavelength candidates must be in a local absorption minimum such as: 800-1350 nm, 1550 nm-1850 nm and 2100 nm-2300 nm. With this system, high-resolution, real-time images of lesion production are possible. Although the patent discusses the imaging of catheter ablation procedures, it does not consider the possibility of constructing the ablation electrode on the endoscope.
The infrared endoscope described in U.S. Pat. No. 6,178,346 contains on its distal end a focusing lens and a hood covering the distal end. The hood is the interface between blood, illumination, and receiving optics in their catheter configurations. One preferred hood embodiment appears as a transparent hemispheric structure extending about 2-4 mm from the optical fibers on the distal end of the catheter. The hoods can be made from various materials including glass, hardened adhesives, polymers, fused silica, and acrylic.
A U.S. patent application was also filed by Amundson, et. al. which applied the infrared endoscope to ablation. The Amundson application teaches means of orienting an extendable ablation electrode so that it is preferentially in contact with tissue instead of blood, guided by visual feedback from the infrared endoscope.
The process of coating a substrate with a transparent or semi-transparent metallic coating has existed for decades. Levy (U.S. Pat. No. 3,515,571) and Kushihashi (U.S. Pat. No. 3,484,263) in 1969-70 describe means of applying thin semi-transparent gold films unto substrates such as glass. O'Brien (U.S. Pat. No. 6,168,825) in a recent patent describes more advanced techniques.
Utilization of a metallic deposit as an ablation electrode has other imaging ramifications. Magnetic resonance (MR) imaging is an alternative to fluoroscopy and has many advantages including soft tissue recognition and no ionizing radiation. Unfortunately, MR imaging is incompatible with conventional ablation catheters due to the powerful magnets in the MR imager. A magnetic material such as stainless steel and platinum (used for the wire connection and electrode in conventional ablation catheters) can be moved or heated by the MR magnets. Only materials with negligible or no para or ferro-magnetism are MR-compatible. Non-magnetic materials include gold, nitinol and titanium. Copper has also been used as a wire material because of its small magnetic properties. The amount of heating and catheter movement on magnetic materials is proportional to the mass of magnetic material in the catheter. When an ablation electrode is formed by coating a non-metallic substrate with a para or ferro-magnetic coating of thickness 20-100 nm, the electrode mass is insignificant and as a result the overall magnetic properties are negligible.